The electromotive forces of a symmetrical concentration cell with transference, Ag; AgCl vertical bar LaCl3 (m*): LaCl3 (m)vertical bar AgCl; Ag, were measured over the concentration range from 8.762 x 10(-4) mol kg(-1) to 6.788 x 10(-2) mol kg(-1) at 298.15 K to obtain the mean activity coefficients of LaCl3. The mean activity coefficient for reference solution at 298.15 K and the ion size parameter for LaCl3 in the extended Debye-Huckel equation are evaluated by using an approach extrapolating concentration to unlimited dilution. A modified Debye-Huckel equation with new parameters has been established for the studied concentration range. A comparison is done of the thermodynamic data of LaCl3 that are determined by this experiment with those reported by previous literatures, and evaluated by some models. (C) 2009 Elsevier B.V. All rights reserved.